Freeze-drying apparatus

ABSTRACT

Freeze-drying apparatus comprising a chamber, means for maintaining a reduced pressure in the chamber, at least one tray defining a receptacle for product to be dried and including a plurality of electrically insulated parallel metallic partitions dividing the receptacle into compartments, means for supplying heat to the product to be dried, and at least one device, for measuring the capacitance and/or the associated resistance of a condenser, connected to at least two of the partitions so as to measure the capacity and/or the associated resistance of a condenser the electrodes of which are constituted by the said partitions.

O United States Patent [1 1 [111 3,811,199 Bouldoires May 21, 1974 [5 FREEZE-DRYING APPARATUS 3,318,012 5/1967 Thompson; 34/92 x [75] Inventor: Jean-Pierre Bouldoires, La Tour-De-Peilz, Switzerland 3:|69:O49 2 9 5 [73] Assignee: Societe DAssistance Technique Pour Produits Nestle S.A., Lousanne, Primary Examiner-Carroll B. Dority, Jr. Switzerland Assistant Examiner-William C. Anderson [22] Filed: Sept. 27, 1972 Y [21 Appl. No.: 292,828 [57] ABSTRACT Freeze-drying apparatus comprising a chamber, means [30] Foreign Application priority Data for maintaining a reduced pressure in the chamber, at s t 30 197] 1' d 4196/7! least one tray defining a receptacle for product to be ep zer dried and including a plurality of electrically insulated parallel metallic partitions dividing the receptacle into (5| compartments means for Supplying heat to the prod 58] Fieid I 5 92 48 uct to be dried, and at least one device, for measuring the capacitance and/or the associated'resistance of a 56] References Cited condenser, connected to at least two of the partitions so as to measure the capacity and/or the associated UNITED STATES PATENTS resistance of a condenser the electrodes of which are 3,605,278 9/1971 Ramsay 34/92 constituted by the said partitions. 3,264,747 8/1966 Fuentevilla 34/92 X 3,343,273 9/1967 Ridge 34/92 X 13 Claims, 4 Drawing Figures mimenwzl m4 f 3.811.199

SHEEI 3 (If 4 1 FREEZE-DRYING APPARATUS The present invention is concerned with a freezedrying apparatus.

ln an industrial freeze-drying operation, the frozen product may be subdivided into granules or similar small particles and deposited on trays which are then placed in an impervious chamber. A reduced pressure is set up in the chamber and heat is supplied to the frozen product to induce sublimation of the ice. The heat energy is generally supplied to the product from the tray which in turn receives this energy by radiation or conduction from heating platens. The tray, preferably made of a metal or alloy which is a good heat conductor such as aluminium or one of its alloys, may have parallel ribs between which the product to be dried is placed, these ribs serving to increase the surface of contact between the product to be dried and the tray.

With a view to improving the yield of freeze-drying operations, and in particular to regulate the heat energy supplied to the product at an optimal level without producing localized melting, efforts have been made to develop methods and means for measuring physical characteristics related to the state of the treated product during freeze-drying. One of these methods, consisting of measuring the dielectric characteristics of the product these are closely related to the amount of ice present permits observation of the evolution of drying (that is the progressive removal of the ice), avoids melting, enables the heat energy supplied to be adjusted at an optimal leval and also gives an accurate indication of the moment when all the ice crystals have disappeared, so that the end of the sublimation period may be detected. This measurement is generally effected by means of a condenser placed within the product to be dried and connected to a bridge for measuring the capacitance and the associated resistance of the condenser, that is the dielectric constant and the leak factor of the product located between the electrodes,-

and allows these to be monitored during freeze'drying. Thus, an alternating voltage applied to a condenser the electrodes of which are separated by a dielectric material induces two electric currents, the first, of strength l w/2 of a phase ahead of the voltage, and the second, of strength 1,, in phase with the voltage, which is the leak current. Therefore, such a condenser may be considered as a pure capacitance in parallel with a resistance, and the values measured are the capacitance and the associated resistance. If the dielectric material of the condenser is the frozen product, and during freezedrying the capacitance and the associated resistance of this condenser are monitored, the progressive elimination of the ice during sublimation provokes a decrease in the dielectric constant of the product, and hence the capacitance, whilst the associated resistance follows the same course. On the other hand, because of the substantial difference between the dielectric characteristics of water and ice, incipient melting is reflected by a very rapid increase of the dielectric characteristics of the product and may be easily detected.

However, the use of a condenser placed within the body of the product to be treated involves many disadvantages, on account firstly of the localized character of the measurement and secondly of the disturbing effect of the condenser itself on the medium. Thus, for obvious practical reasons, the condenser should be of small dimensions, particularly if the tray has ribs defining compartments, and the sample observed represents only a very small fraction of a product which itself is hardly homogeneous On the other hand, the condenser itself disturbs heat transfer within the product and modifies its degree of compaction. ln view of this, the drying conditions of the product situated between the electrodes of the condenser may be notably different from those of the rest and the sample observed is not representative of the total product. Finally, the insertion of a measuring cell even of reduced size into the product to be treated is a delicate operation hardly compatible with the operating criteria of an industrial apparatus.

The object of the present invention is to provide a freeze-drying apparatus that is particularly simple to operate and which permits monitoring of the dielectric characteristics of the product to be treated without disturbing the drying conditions. The freeze-drying apparatus according to the invention comprises an impervious chamber, means for maintaining a reduced pressure in the chamber, at least one tray defining a receptacle for product to be dried and including a plurality of electrically insulated parallel metallic partitions dividing the receptacle into compartments, means for supplying heat to the product to be dried, and at least one device, for measuring the capacitance and/or the associated resistance of a condenser, connected to at least two of the partitions so as to measure the capacity and/or the associated resistance of a condenser the electrodes of which are constituted by the partitions.

In one preferred embodiment of the invention, the apparatus comprises an impervious chamber, means for maintaining a reduced pressure in the chamber, such as a pump and a condenser of known type, and one or more trays disposed in the chamber. Each tray may comprise a rigid body of electrically nonconducting material, for example a thermosetting plastic. The rigid body preferably consists of a rectangular frame and a flat bottom defining a receptacle in the form of a right-angled parallelepiped. The bottom is traversed by two series of tubes, made of insulating material, the free ends of which protrude upwards from the inner side of the bottom so as to provide outlets which are evenly spaced on two straight paths parallel and in proximity to the short sides of the frame. These two series of tubes are each connected to a manifold itself connected to a circuit for circulating an electrically non-conducting heating fluid, for example glycol. The partitions in this embodiment are hollow metallic members, made, for example, by extrusion with the open ends blocked, having two flat parallel sides and a longitudinal axis perpendicular to the two paths defined bythe tube outlets. The side of each partition in contact with the bottom of the tray has two holes in which tube outlets are inserted, one from each series of tubes. The partitions thus form channels closing in parallel the heating fluid circuit. These partitions are electrically connected to a measuring device comprising a bridge and a source of alternating current. The partitions are preferably connected in such a way that the adjacent condensers formed by pairs of partitions are connected in parallel with the measuring device which itself is located outside the chamber. The bridge of the measuring device is preferably balanced automatically, allowing continuous measurement and recording of the dielectric characteristics of the product during freezedrying. It is also possible to incorporate the measuring device in an automatic system regulating the heat supply, responding, for example, by comparison of the measured values with previously defined reference values. The apparatus may comprise several measuring devices connected to different trays, or a single device connected with the tray which is considered to be the most representative. It is possible also to use an electric commutator for the measurement of the characteristics of a specific part of the product contained in a tray, or to record separately these characteristics for certain compartments only.

Alternatively, the tubes in which the heating fluid flows may pass through the sides of the frame, entering the partitions through the ends. Two such partitions may also provide two of the sides of the frame.

It is also possible, as is known, to insert screens at the bottom of each compartment of the tray for facilitating the removal of water vapour produced by sublimation.

According to the second embodiment of the invention, at least one of the trays comprises a rigid metallic body and the partitions are parallel metal strips in contact with the body of the tray, one at least of the contact surfaces between the body and each strip being coated with an electrically insulating material. In this second embodiment, the heat energy is supplied to the product to be treated by way of the tray which itself receives this energy from heating platens, either by conduction if the heating platens are in contact with the tray, or by radiation.

The body of the tray, which is preferably made of a metal or alloy which is a good heat conductor such as aluminium, comprises a rigid rectangular frame and a flat bottom defining a receptacle in the form of a rightangled parallelepiped. The metal strips constituting the partitions of the tray are parallel to two sides of the frame and rest on the bottom, defining parallelepipedshaped compartments. Two sides of the frame may also be constituted by these partitions. The extremities of these strips are held in grooves milled in the inner faces of the two other sides of the frame, and a screen for fa cilitating removal of water vapour may also be placed in the bottom of each compartment.

The electrical connection of the partitions to the measuring device is identical to that previously described, and may be varied in the same way. 7

According to one variant of this embodiment, the parts of the metal strips which are in contact with the body of the tray, that is the ends held in the grooves in the frame'and the lower edges of these strips resting on the bottom, are coated with a thin layer of an electrically insulating material such as a varnish or a polyester, for example a sheet of Mylar (Registered Trade Mark). The thickness of the insulating layer, though sufficient to insulate the strips electrically from the body of the tray, is nevertheless very small and hardly modifies the heat transfer between the body and the strips. In an alternative, particularly preferred construction, the strips forming the partitions are made of aluminium or one of its alloys, and their edges in contact with'the body of the tray are coated with a thin layer of alumina obtained by anodic oxidation. This alumina layer, having a thickness of the order of to 10 microns, has practically no effect on heat exchange by conduction between the body of the tray and the partitions, but assures electrical insulation of the partitions. Furthermore, by virtue of the excellent adhesion of the alumina layer to aluminium and the extreme hardness of alumina, the risks of abrasion of the insulating layer, which could be present during insertion of the strips in the grooves in the frame, are negligible and the strips may be fitted in the grooves with very little play to provide good thermal contact.

According to the second variant of this second embodiment, the strips may be entirely coated with an electrically insulating material. This variant is particularly advantageous if the strips are made of aluminium and have been treated by an anodic oxidation which blackens the alumina layer formed. The blackening, it has been observed, improves the absorption by the partitions of the heat energy radiated by the heating platens.

According to the third variant of this second embodiment, the metal strips constituting the partitions are bare and the parts of the metallic body of the tray in contact with the strips are coated with an electrically insulating material. ln this variant, the insulating material is deposited on the bottom and on the inside faces of the sides of the metal frame, in parallel bands slightly wider than the thickness of the strips. If the tray is provided with screens for facilitating removal of water vapour produced by sublimation, it is advantageous to use a bottom made by extrusion according to known methods and having parallel ribs of which the height is approximately equal to the distance between the bottom and the screen, the upper faces of these ribs being coated with an electrically insulating material. Likewise, parallel ribs may be provided on the inside walls of the frame at the points of contact with the strips, the contact surfaces of the ribs being coated with an insulating material. In this case, the ribs are provided with clips made of a resilient insulating material such as a plastic, for correct placing of the partitions.

In the fourth variant of this second embodiment, the body of the'loading tray is entirely coated with a layer of an electrically insulating material. This variant has been found to be particularly advantageous when the body is made of aluminium or an aluminium alloy and has been treated by anodic oxidation. If the inner faces of the frame and/or the bottom of the tray are provided with grooves for engaging the strips, the hardness of the superficial alumina layer permits the strips to be fitted with very little play so that excellent contact affording very good heat conduction between the body of the tray and the partitions is obtained. Furthermore, if the tray is heated by radiation, it is possible to blacken the oxidised surface which increases considerably the absorption coefi'icient of the tray and improves heat transfer.

The accompanying drawing shows, by way of example, two preferred embodiments of the freeze-drying apparatus according to the invention.

HO. 1 is a schematic plan view of the apparatus according to the first embodiment;

FIG. 2 is a partial section along ll.-ll of a tray of the apparatus shown in FIG. 1;

FIG. 3 is a schematic plan view of the apparatus according to the second embodiment; and

FIG. 4 is a perspective view of a tray of the apparatus shown in H6. 3.

As shown in FIGS. 1 and 2, the freeze-drying apparatus according to the invention comprises an impervious chamber 1 communicating with a condenser 2 and a vacuum pump 3. The chamber 1 contains a series of super-imposed trays 4, only the upper tray being visible in FIG. 1. Each tray 4 comprises a body 5 made of an electrically non-conducting thermo-setting plastic material defining a receptacle in the form of a right-angled parallelepiped and consisting of a bottom 6 and a rectangular frame 7. The bottom is traversed by two series of tubes, 8 and 9 respectively, these tubes being made of an electrically insulating material, and each series of tubes is connected to a manifold 10, respectively 11. These manifolds are connected to a circuit 12 for circulating heated glycol which comprises a heat exchanger 13 for heating the glycol and a circulating pump 14. The free ends of the tubes 8 and 9 protrude upwards from the inner side of the bottom 6, and form outlets evenly spaced on two straight paths parallel to the short sides of the frame 7. The tray 4 also comprises partitions 15 consisting of hollow metallic members in the form of parallelepipeds the cavity of which is in communication with the two series of tubes 8 and 9, and each partition constitutes a circulation element for glycol between two tubes one each from the series 8 and 9. The partitions 15 of one of the trays are electrically connected in parallel to the measuring device 16 located outside the chamber, by leads l7 and 18 which pass through an impervious passage 19 in the wall of the chamber 1. On the other hand, in each compartment defined by the partitions 15 is placed a screen 20 made of insulating material for facilitating the passage and removal of the water vapour emitted from the product during freeze-drying.

The partitions 15, which serve to heat the product, also act as the electrodes of adjacent plate condensers connected in parallel to the measuring device 16.

As shown in FIGS. 3 and 4, the freeze-drying apparatus according to the invention likewise comprises an impervious chamber 1 communicating with a condenser 2 and a vacuum pump 3. This chamber contains a series of super-imposed trays 21, only the upper tray being visible in FIG. 3. Each tray comprises a metallic body 22 made up of a rectangular frame 23 and a bottom 24 defining a receptacle in the form of a rightangled parallelepiped. The partitions 25 which divide this receptacle into compartments are flat aluminium strips coated with a black alumina layer 7 microns in thickness obtained by anodic oxidation. The strips 25 are held in grooves 26 milled in the inner faces of the short sides of the frame 23 and rest on the bottom 24. Screens 27 are placed in each compartment defined by the partitions 25 to facilitate removal of water vapour.

The apparatus also comprises heating platens 28 located within the chamber 1, between the difi'erent loading trays. These platens, supplied with hot glycol by a circuit comprising a heat exchanger 13 and a circulating pump 14, supply heat energy by radiation. The energy radiated by the platen situated under each tray is absorbed by the body 22 of the tray and is transmitted to the partitions 25 by conduction between the frame 23 and the partitions. These partitions also absorb energy radiated by the heating platen located above each tray.

The partitions 25 of each tray 21 are electrically connected in parallel to a measuring device 16 located outside the chamber l, by two leads l7 and 18 passing through the wall of the chamber 1 by an impervious passage 19.

What is claimed is:

l. Freeze-drying apparatus comprising a chamber, means for maintaining a reduced pressure in the chamber, at least one tray defining a receptacle for product to be dried and including a plurality of electrically insulated parallel metallic partitions dividing the receptacle into compartments, means for supplying heat to the product to be dried, and means connected to at least two of the partitions for measuring the electrical current flow characteristics of a condenser, the electrodes of which are constituted by the said partitions.

2. Apparatus according to claim 1 in which the means for supplying heat to the product include means for heating the partitions.

3. Apparatus according to claim 2, in which the tray comprises a body made of an electrically insulating material and the partitions are hollow members the cavities of which are in communication with a circuit of an electrically non-conducting heating fluid.

4. Apparatus according to claim 2, in which atleast one heating platen is provided within the chamber for supplying heat to the tray, the tray comprises a rigid metallic body and the partitions are parallel strips in contact with the body of the tray, at least one of the surfaces of contact between the body of the tray and each strip being coated with an electrically insulating material.

5. Apparatus according to claim 4, in which the strips are made of aluminium or an alloy thereof.

6. Apparatus according to claim 5, in which at least the surfaces of the strips in contact with the tray body are coated with a layer of alumina obtained by anodic oxidation.

7. Apparatus according to claim 6, in which the strips are entirely coated with a layer of black alumina.

8. Apparatus according to claim 4, in which the tray body is made of aluminium or an alloy thereof.

9. Apparatus according to claim 8, in which at least the surfaces of the body in contact with the strips are coated with a layer of alumina obtained by anodic oxidation.

10. Apparatus according to claim 5, in which the electrically insulating material is a polymer.

11. Apparatus according to claim 4, in which the tray body comprises a flat bottom and two parallel sides perpendicular to the bottom the facing surfaces of which have parallel grooves perpendicular to the bottom and the ends of the strips are held in these grooves.

13. Apparatus according to claim 1, in which said measuring means includes first means for applying an alternating voltage to said partitions and second means for measuring current in phase with said voltage. 

1. Freeze-drying apparatus comprising a chamber, means for maintaining a reduced pressure in the chamber, at least one tray defining a receptacle for product to be dried and including a plurality of electrically insulated parallel metallic partitions dividing the receptacle into compartments, means for supplying heat to the product to be dried, and means connected to at least two of the partitions for measuring the electrical current flow characteristics of a condenser, the electrodes of which are constituted by the said partitions.
 2. Apparatus according to claim 1 in which the means for supplying heat to the product include means for heating the partitions.
 3. Apparatus according to claim 2, in which the tray comprises a body made of an electrically insulating material and the partitions are hollow members the cavities of which are in communication with a circuit of an electrically non-conducting heating fluid.
 4. Apparatus according to claim 2, in which at least one heating platen is provided within the chamber for supplying heat to the tray, the tray comprises a rigid metallic body and the partitions are parallel strips in contact with the body of the tray, at leaSt one of the surfaces of contact between the body of the tray and each strip being coated with an electrically insulating material.
 5. Apparatus according to claim 4, in which the strips are made of aluminium or an alloy thereof.
 6. Apparatus according to claim 5, in which at least the surfaces of the strips in contact with the tray body are coated with a layer of alumina obtained by anodic oxidation.
 7. Apparatus according to claim 6, in which the strips are entirely coated with a layer of black alumina.
 8. Apparatus according to claim 4, in which the tray body is made of aluminium or an alloy thereof.
 9. Apparatus according to claim 8, in which at least the surfaces of the body in contact with the strips are coated with a layer of alumina obtained by anodic oxidation.
 10. Apparatus according to claim 5, in which the electrically insulating material is a polymer.
 11. Apparatus according to claim 4, in which the tray body comprises a flat bottom and two parallel sides perpendicular to the bottom the facing surfaces of which have parallel grooves perpendicular to the bottom and the ends of the strips are held in these grooves.
 12. Apparatus according to claim 1, in which said measuring means includes first means for applying an alternating voltage to said partitions and second means for measuring current of a phase ahead of said voltage.
 13. Apparatus according to claim 1, in which said measuring means includes first means for applying an alternating voltage to said partitions and second means for measuring current in phase with said voltage. 